Active pixel sensors, such as described in U.S. Pat. No. 5,471,215, enable acquisition of images at relatively higher rates. Different pixel circuits have been designed which enable acquiring images at even higher rates. This can enable obtaining image information after a short time of integration, and hence allow very fast snapshots to be taken with such a sensor. The integration times for such a sensor may be considerably shorter than the time it takes to read out a frame, called the “frame readout time”. Fast-moving images, therefore, are effectively frozen in time.
A limit on the speed of such a system may be set by the time it takes to read out the entire active pixel array, which may include more than one million pixels. This read out time is typically at least 100 times the length of the frame integration time.